1. Field of the Invention
The present invention relates to the telecommunication field and in particular to the transport of Ethernet frames over a transport SDH/SONET network. Still more in particular, the present invention relates to a method and device for providing a minimum delay dispatching of Ethernet traffic frames over the SDH/SONET network.
As it is known, traffic generated by an Ethernet apparatus is characterized by discontinuities, namely there are periods with a more or less constant sending rate of Ethernet packets and periods during which a rather long time is provided between a received Ethernet frame and the next one. Such unstable/inconstant traffic is generally termed “bursty”. On the contrary, SDH or SONET traffic is characterized by a constant sending/receiving rate. In other words, any network element of a transport SDH/SONET network sends corresponding frames with a regular and constant rate. Furthermore, Ethernet frames do not have a fixed length/size but only a maximum size (1518 bytes).
It is easy to understand that these discrepancies result in a highly difficult interfacing of two technologies having different natures/characteristics.
2. Description of the Prior Art
An already available solution to the above problem allows the mapping of Ethernet frames into SDH/SONET Virtual Containers as a transparent tributary; all incoming bits are transported to the output interface with the related timing information (frequency for recovering the proper bit rate at the reception side). Within the SDH/SONET payload also the dead times between a received Ethernet frame and the following one are mapped.
The general problem of transporting Ethernet frames over a SONET/SDH transport network is presently solved through SONET/SDH virtual concatenation. Ethernet frame transport is performed according to the following main steps: the bytes of one frame are distributed among all the available SDH/SONET Virtual Containers, namely, the first frame byte is mapped in the first VC, the second frame byte is mapped in the second VC and so on; due to the fact that SDH/SONET Virtual Containers can follow different paths, at the ending point, the Virtual Containers should be realigned; and the bytes of the Ethernet frames are extracted from the realigned Virtual Containers and the frame is finally re-assembled.
At present, when Ethernet traffic is transported over SDH/SONET networks, some queues of Ethernet frames are required. The Ethernet frames should be transported over SDH/SONET network through point-to-point connections. As a point-to-point connection could be accomplished by means of different routes connecting the starting and ending points, the problem to solve is to find the best route in terms of transport and network performances.
As far as the inventors are aware, there is no known solution to this problem and the frames are transmitted through fixed point-to-point connections, regardless whether the resources which are intended for the transport are overused and some other resources (which could provide a reduced dispatching time) are underused.
The above problem could become even worst when there is a fault of the SDH/SONET Virtual Containers transporting the Ethernet frames. At present, faults are managed according to the relevant SDH/SONET Recommendations but, in many case, a fault affecting a Virtual Container assigned to the transport of Ethernet frames leads to a complete loss of the traffic.